Native lignin is a naturally occurring amorphous complex cross-linked organic macromolecule that comprises an integral component of all plant biomass. The chemical structure of lignin is irregular in the sense that different structural units (e.g., phenylpropane units) are not linked to each other in any systematic order. It is known that native lignin comprises pluralities of two monolignol monomers that are methoxylated to various degrees (trans-coniferyl alcohol and trans-sinapyl alcohol) and a third non-methoxylated monolignol (trans-p-coumaryl alcohol). Various combinations of these monolignols comprise three building blocks of phenylpropanoid structures i.e. guaiacyl monolignol, syringyl monolignol and p-hydroxyphenyl monolignol, respectively, that are polymerized via specific linkages to form the native lignin macromolecule.
Extracting native lignin from lignocellulosic biomass during pulping generally results in lignin fragmentation into numerous mixtures of irregular components. Furthermore, the lignin fragments may react with any chemicals employed in the pulping process. Consequently, the generated lignin fractions can be referred to as lignin derivatives and/or technical lignins. As it is difficult to elucidate and characterize such complex mixture of molecules, lignin derivatives are usually described in terms of the lignocellulosic plant material used, and the methods by which they are generated and recovered from lignocellulosic plant material, i.e. hardwood lignins, softwood lignins, and annual fibre lignins.
Native lignins are partially depolymerized during the pulping processes into lignin fragments which are soluble in the pulping liquors and subsequently separated from the cellulosic pulps. Post-pulping liquors containing lignin and polysaccharide fragments, and extractives, are commonly referred to as “black liquors” or “spent liquors”, depending on the pulping process. Such liquors are generally considered a by-product, and it is common practice to combust them to recover some energy value in addition to recovering the cooking chemicals. However, it is also possible to precipitate and/or recover lignin derivatives from these liquors. Each type of pulping process used to separate cellulosic pulps from other lignocellulosic components produces lignin derivatives that are very different in their physico-chemical, biochemical, and structural properties.
Given that lignin derivatives are available from renewable biomass sources there is an interest in using these derivatives in certain industrial processes. For example, U.S. Pat. No. 5,173,527 proposes using lignin-cellulosic materials in phenol-formaldehyde resins. A. Gregorova et al. propose using lignin in polypropylene for it radical scavenging properties (A. Gregorova et al., Radical scavenging capacity of lignin and its effect on processing stabilization of virgin and recycled polypropylene, Journal of Applied Polymer Science 106-3 (2007) pp. 1626-1631). However, large-scale commercial application of the extracted lignin derivatives, particularly those isolated in traditional pulping processes employed in the manufacture of pulp and paper, has been limited due to, for example, the inconsistency of their chemical and functional properties. This inconsistency may, for example, be due to changes in feedstock supplies and the particular extraction/generation/recovery conditions. These issues are further complicated by the complexity of the molecular structures of lignin derivatives produced by the various extraction methods and the difficulty in performing reliable routine analyses of the structural conformity and integrity of recovered lignin derivatives.
Carbon fibres are known to have certain mechanical and physio-chemical properties that make them useful in many applications. For example, carbon fibres may have high tensile strength, low density, low weight, and/or low thermal expansion. Individual strands of carbon fibres can be twisted together to form a yarn that can be used by itself or woven into fabrics. Carbon fibre yarns can also be combined with plastic resins that can be wound or molded to form composite materials such as carbon fibre-reinforced plastics. However, while carbon fibre-containing composites may have certain advantages over similarly sized steel materials, they are usually much more costly because of the high cost of manufacturing carbon fibres.
Carbon fibres are generally manufactured by carbonization of polymerized acrylonitrile (polyacrylonitrile). Polyacrylonitrile is converted to carbon fibres with a multistep process wherein the first step is heating the polyacrylonitrile to 300° C. to break the hydrogen bonds and add oxygen molecules thereby creating a fireproof and stable material. This new material is then carbonized by heating to between 1,500° C. and 3,000° C. in an inert gas resulting in a material that comprises almost 100% carbon. The carbonized material is then surface-treated and sized with an epoxy resin to protect the carbon fibre. Different grades of carbon fibre can be produced by selection of the temperatures for carbonization. For example, carbon fibres that have very high tensile strengths are formed at temperatures between 1,500° C. to 2,000° C. degrees, Carbon fibres with high modulus (i.e., more elasticity) are produced by carbonization at higher temperatures, e.g., up to 3,000° C.
Lignin derivatives recovered from kraft pulping processes (i.e., commercial kraft lignins) and from organosolv processes have been evaluated for production of low-cost carbon fibre that may be used to partially or completely replace carbon fibres produced from polyacrylonitrile. For example, U.S. Pat. No. 3,461,082 proposed a method for producing a carbonized lignin fiber from alkali-lignins, thiolignins, or ligninsulfonates. J. F. Kadla et al. proposed using commercial kraft lignin for production of carbon fibres (J. F. Kadla et al., 2002, Lignin-based carbon fibers for composite fiber applications, Carbon 36: 1119-1124). S. Kubo et al. proposed using acetic acid organosolv lignin for production of carbon fibres (S. Kubo et al., 1998, Preparation of Carbon Fibers from Softwood Lignin by Atmospheric Acetic Acid Pulping, Carbon 36: 1119-1124). Unfortunately, in each of these systems, production costs have not significantly decreased because of the purification steps required to remove volatiles, ash and particulates. Furthermore, purified lignins required the addition of co-polymers and plasticizers to form carbon fibres. S. Kubo et al. proposed using Alcell® organosolv lignin for production of carbon fibres (S. Kubo et al., 2004, Poly(Ethylene Oxide)/Organosolv Lignin Blends: Relationship Between Thermal Properties, Chemical Structures, and Blend Behaviour, Macromolecules 37: 6904-6911). However, they found that while a small amount of Alcell® lignin increased the crystallinity of poly(ethylene oxide), incorporating more than 25% Alcell® lignin hindered crystallinity and crystalline domain size. Other investigators have suggested using lignin derivatives in carbon fibre compositions. See, for example, U.S. Pat. No. 6,765,028; WO2009/028969; U.S. Pat. Nos. 7,678,358; 5,344,921; US2010/0311943.